Heat treating method

ABSTRACT

Heat treating apparatus which includes a heat treating furnace, a chute at the back of the furnace, and a quench bath at the bottom of the chute. The chute has at least one opening in it located at a level above the quench bath, through which a purging gas is injected, and at a slightly higher level the chute has at least one opening in it through which purging gas, furnace atmosphere gas, quench bath fumes and other contaminants are exhausted. Also included is a method of heat treating objects utilizing the apparatus.

United States Patent Keough 51 Mar. 21, 1972 [54] HEAT TREATING METHOD William R. Keough, Bloomfield Hills, Mich.

[73] Assignee: Multiiastener Company, Detroit, Mich.

[22] Filed: Oct. 27, 1969 [21] Appl. No.: 869,524

[72] Inventor:

[52] U.S.C1 ..148/l43, 148/16, 148/165, 148/166, 148/20.3, 148/153 [51] Int. Cl. ..C2ld 1/00 [58] Field of Search ..l48/143,153,13,20.6,18, 148/16, 16.5, 16.6, 20.3

[56] References Cited UNITED STATES PATENTS Bechtel et a1 148/143 Primary Examiner'Richard 0. Dean Attorney-Settle, Batchelder & Oltman [5 7] ABSTRACT Heat treating apparatus which includes a heat treating furnace, a chute at the back of the furnace, and a quench bath at the bottom of the chute. The chute has at least one opening in it located at a level above the quench bath, through which a purging gas is injected, and at a slightly higher level the chute has at least one opening in it through which purging gas, furnace atmosphere gas, quench bath fumes and other contaminants are exhausted.

Also included is a method of heat treating objects utilizing the apparatus.

4 Claims, 3 Drawing Figures PAIENTEDMARZI I972 3,650,853

SHEET 2 [1F 2 l N VEN TOR WILLIAM R. KEOUGH SETTLE, BATCHELDER 8 OLTMAN.

ATT'YS.

HEAT TREATING METHOD 1. Field of the Invention This invention relates to a heat treating furnace and associated apparatus of a type having a chute located at the back of the heating chamber of the furnace which serves as a conduit for channeling heat treated metal objects into a liquid quench bath. After the heat treating step is completed, the objects being treated are automatically dumped down through the chute into the quench bath where isothermal transformation of the metal occurs.

A nonoxidizing atmosphere is ordinarily used in the furnace to prevent decarbonization of the metal objects being treated, or in some instances to carburize them.

2. The Problem It is important in furnaces of the above described type, that the composition of the nonoxidizing atmosphere be rigidly controlled to obtain consistent results in heat treating. Any

contaminants present in the atmosphere will affect the decarbonization or carburization of the metal objects being treated, and thus ultimately afiect the hardness of them.

Molten salt is commonly used as a quench bath, and is normally heated to a temperature of 600-700 F. At that temperature the salt has a rather strong vapor pressure, thus fumes, vapor and the like from the quench bath drift up the aforementioned chute into the heating chamber of the furnace and contaminate, to a certain extent, the nonoxidizing atmosphere. The fumes also erode the metal alloy or stainless steel conveyor belts used in transporting objects through the furnace.

Additionally, when natural gas, or other carburizing gases such as carbon monoxide, ethane and propane, is used in the nonoxidizing atmosphere, all of it does not react with the metal objects being treated, but rather some of it breaks down and snows out" or precipitates, liberating small carbon particles in the form of soot. If a significant quantity of these soot particles should happen to drop into the quench bath they could cause an explosion due to the heat of reaction with the molten salt.

Still further, water is sometimes added to a quench bath to increase the severity of the quench, and to increase the fluidity of the bath, thus minimizing the amount of quench bath fluid which adheres to the objects being treated when they are removed from the bath. Most of the water introduced into the bath vaporizes out. When the water vapor enters into the furnace heating chamber, it reacts with methane in the carburizing gas to form carbon dioxide and hydrogen, with the result that the carburizing gas burns out.

It is therefore necessary to keep the fumes given off by the quench bath from contaminating the atmosphere in the furnace and keep precipitated carbon particles in the furnace atmosphere from dropping into the quench bath.

An object of this invention is to provide a furnace construction which will minimize the contamination of the furnace atmosphere by quench bath fumes, and which will substantially eliminate the change of precipitated carbon from the furnace atmosphere dropping into the quench bath.

It is also an object of this invention to provide a heat treating process which will substantially eliminate the change of precipitated carbon from the furnace atmosphere from dropping into the quench bath.

Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.

ON THE DRAWINGS FIG. 1 is a side elevation view, partly in section, of the heat treatment apparatus of the invention;

FIG. 2 is a transverse cross-sectional view taken on line 2- 2 ofFlG. l; and

FIG. 3 is an enlarged cross-sectional view of the chute leading from the heat treating furnace to the quench bath tank with parts broken away.

SUMMARY OF THE INVENTION contaminants are exhausted from the chute at a rate sufficient to prevent fumes from the quench bath entering into the heating chamber of the furnace, and to prevent soot from dropping into the quench bath.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, there is shown a heat treatment furnace 10 of more or less conventional construction which takes the form of a thermally insulated tunnellike heating chamber 12, through which objects to be heated are conveyed on the upper run of an endless conveyor belt 14. The belt 14 is trained around a drive roller 16 and an opposite end roller 18, and is driven by a motor (not shown) in a direction such that the upper run of the belt moves from the left to right as viewed in FIG. 1. Heat is imparted to the heating chamber 12 by means of a plurality of burner tubes 20 connected to burner pots (not shown). The temperature within the chamber 12 and the speed of movement of the objects are cooperatively adjusted to achieve the desired heat exposure during their transit through the chamber 12. The furnace 10 has an inlet opening or entrance 22,- and a chute 24 surrounding the opening 22 depending toward the forward end of the upper run of the conveyor belt 14.

A gas inlet tube 26 is mounted in the top of the furnace 10, opening into the heating chamber 12. A fan 28 driven by a motor 30 is also mounted in the top of the furnace 10 to circulate the gas being injected through the gas inlet tube 26.

The controlled atmosphere gas most commonly used is a high carbon potential carburizing gas comprising about 19.9% C0, 38.7% H 58% CH 0.2% CO and the balance nitrogen.

Other carburizing gases can be used. In general these contain a relatively high proportion of carbon monoxide, methane, ethane or propane. To minimize the deposition of free carbon particles, or soot, the gases are conventionally diluted with nitrogen or hydrogen. By suitable adjustment of temperature, and gas composition, the surface carbon content and carbon gradient of the objects to be treated can be varied to meet almost any requirement.

Nomoxidizing gases are also used to prevent decarbonization of the metal objects being treated. Gases which are designed to simply prevent decarbonization usually have a low carbon content.

The gas which is supplied to the heating chamber 12 through the tube 26 may also be ammonia for nitriding purposes. In general, any desired gas may be introduced into the heating chamber 12 to provide a controlled atmosphere, and the gas may be any desired temperature.

Objects to be conveyed through the furnace are fed from an infeed conveyor 32 through the entrance 22 and the chute 24 onto the upper run of the belt 14.

When the objects reach the right hand end of the conveyor, as viewed in FIG. 1, they drop freely from the belt 14 downward through an outlet opening 34 in the floor of the heating chamber 12 and through a rectangular chute 36 which extends downwardly from the opening 34 into a quench bath tank 38. A quench bath of molten salt, or oil in some instances, iscontained within the tank 38. A suitable tank is described in US. Pat. No. 3,442,274, and is hereby incorporated by reference.

The lower end of the chute 36 has a horizontal slot 40 (see FIG. 3) cut into it around the periphery thereof. A manifold 42 is mounted around the outside of the chute 36 and encloses the slot 40. A conduit 44 connects the manifold 42 to a compartment 46 containing quench bath liquid, located along a wall of the quench bath tank 38. Quench bath liquid is pumped from the compartment 46, through the conduit 44 and into the manifold 42. The liquid is discharged from the manifold 42 into the chute 36 through the slot 40. Streams from opposite sides of the chute intersect each other in approximately the center of the chute 36. The streams serve as a liquid curtain to prevent liquid in the quench bath tank 38 from splashing up the chute 36 into the heating chamber 12 when objects are dropped from the conveyor belt 14 into the quench bath tank 38. The liquid curtain and its function is also described in U.S. Pat. No. 3,442,274. The liquid curtain, quench bath tank, and associated apparatus form no part of this invention.

A conveyor 48 is situated in the bottom of the quench bath tank 38 for receiving objects falling through the chute 36. The conveyor 48 transports the objects to a water bath 50 where they are washed and further cooled.

Referring to FIG. 3, the chute 36 has a horizontal slot 52 through which a purging gas is injected, cut into it around the periphery thereof at a level above the level of the slot 40 through which quench bath liquid is flowing. A manifold 54 containing purging gas is mounted around the outside of the chute 36, enclosing the slot 52. The manifold 54 is connected by means of a conduit 56 to a source of the purging gas.

The chute 36 has still another horizontal slot 58 cut into it around the periphery thereof at a level above the level of the slot 52 through which purging gas is injected. Purging gas, quench bath fumes, and other contaminants are exhausted through the slot 58 into a manifold 60 mounted around the outside ofthe chute 36 and enclosing the slot 58. A conduit 62 leading from the manifold 60 delivers the exhausted gases to the external atmosphere. They are burnt off at the mouth of the conduit. An exhaust pump or eductor (schematically shown at 63) is utilized to pull the purging gas through the conduit 62. An inwardly projecting skirt 64 is attached to the interior wall of the chute 36 at a level above the slot 58 to direct fumes, gases and the like, into the exhaust slot 58.

A plurality of spaced apart slots or orifices around the periphery of the chute 36 can be used instead of the single slots 52 and 58.

When a high carbon potential carburizing gas is being used for the controlled atmosphere in the heating chamber thermic gas (e.g., 40% H 20.9% C0, 40% N This is injected into the chute through the slot 52, preferably at a rate of about 400 cubic feet per hour. The endothermic gas contains a low amount of carbon as compared to the high carbon potential gas normally used in the heating chamber of the furnace.

The carbon potential ofa gas is measured with reference to its dew point. The high carbon potential gases have a relatively low dew point, while the low carbon potential gases have a relatively high dew point. For the purposes of this invention a high carbon potential gas can be defined as any carbon containing gas having a dew point of about 70 F. or lower. A low carbon potential gas can be defined as any carbon containing gas having a dew point substantially higher than that of the high carbon potential gas.

The preferred high carbon potential gas used in the heating chamber may have a dew point as low as F. and thus has a tendency to precipitate out carbon when it cools. For example, in the treatment of steel having 0.70 percent carbon, the dew point is controlled at 30-35 F. when the furnace is operating at 1,600" F. In a steel having 1.00 percent carbon the dew point will be about 20F. at the same temperature in the furnace. The low carbon potential endothermic purging gas, on the other hand, has a dew point in excess of the specific carbon potential gas preferably at least about 70 F. and does not precipitate out carbon when it cools. The terms high carbon potential gas and low carbon potential gas thus are comparative, the dew point of the high carbon gas depending upon the carbon content of the steel being treated and the low carbon gas having a dew point dependent upon the specific high carbon gas utilized.

Thus injecting the low carbon potential purging gas into the chute and mixing it with the high carbon potential gas has the effect of raising the dew point of the high carbon potential gas and preventing carbon from precipitating out. This, coupled with the exhausting of all gases and contaminants through the slot 58 effectively prevents precipitated carbon from falling into the quench bath.

Any other low carbon potential purging gas can be used in conjunction with the high carbon potential gas used for the furnace surface atmosphere to prevent precipitated carbon from falling into the quench bath. Other suitable gases include, for example, carbon dioxide.

Purging gases other than the low carbon potential gas can be used when carbon precipitating gases are not used as the controlled atmosphere in the heating chamber. The specific nature of this purging gas is not critical, as long as it is nonoxidizing.

Injected gas, quench bath fumes, and other contaminants are exhausted sufficiently to prevent quench bath fumes from entering the heating chamber and precipitated carbon particles from falling into the quench bath. Preferably exhaust gases are drawn through the slot 58 at a rate at least two times the rate purging gas is injected into the chute 36 through the slot 52. In a preferred embodiment it is exhausted at the rate of 800 cubic feet per hour. Some controlled atmosphere gas in the heating chamber will be drawn out at the same time.

Exhaustion of the gases and quench bath fumes through the slot 58 serves the dual function of preventing contamination of the furnace atmosphere and erosion of the belt by quench bath fumes, and also prevents the precipitating carbon from unreacted gas from falling into the quench bath.

In the event a controlled atmosphere which does not precipitate soot is being used in the furnace chamber 12, gases need be exhausted through the slot 58 in the chute 36 at a rate sufficient only to prevent quench bath fumes from rising through the chute 36 into the heating chamber 12.

What I claim is:

1. In the method of heat treating objects which includes transporting objects through the heating chamber of a heat treating furnace, and thereafter feeding the objects down through a chute into a quench bath giving off fumes, the improved method of preventing contaminating particles from the furnace from falling through the chute to the bath comprising (1) injecting a continuous stream of purging gas into the chute through a lateral opening above the level of the quench bath and (2) exhausting purging gas, fumes and other contaminants from the chute at a level above the level of injection of the purging gas; the purging gas, fumes, and other contaminants being exhausted at a rate sufficient to preclude fumes from the quench bath from rising through the chute into the heating chamber.

2. In the method of heat treating objects which includes transporting objects through a heating chamber having a controlled atmosphere, and thereafter feeding the objects down through a substantially vertical chute having the same controlled atmosphere, into a quench bath giving off fumes, the improved method of preventing contaminating particles from the furnace from falling through the chute to the bath comprising (l) injecting a continuous stream of purging gas into the chute through a substantially horizontal opening in the generally vertical chute and (2) exhausting gas, fumes and other contaminants from the chute at a level directly above the level of injection of the purging gas; the gas, fumes and other contaminants being exhausted at a rate sufficient to preclude contaminants from falling into the quench bath.

3. The method of claim 2 in which the controlled atmosphere is a high carbon potential carburizing gas, the purging gas is a low carbon potential gas, and in which the gas, fumes and other contaminants are exhausted from the chute at a rate equal to about two times the rate the purging gas is injected.

4. The method of claim 3 in which the purging gas is injected into the chute at the rate of about 400 cubic feet per minute, and the gas, fumes, and other contaminants are ex- 5 hausted at about 800 cubic feet per minute. 

2. In the method of heat treating objects which includes transporting objects through a heating chamber having a controlled atmosphere, and thereafter feeding the objects down through a substantially vertical chute having the same controlled atmosphere, into a quench bath giving off fumes, the improved method of preventing contaminating particles from the furnace from falling through the chute to the bath comprising (1) injecting a continuous stream of purging gas into the chute through a substantially horizontal opening in the generally vertical chute and (2) exhausting gas, fumes and other contaminants from the chute at a level directly above the level of injection of the purging gas; the gas, fumes and other contaminants being exhausted at a rate sufficient to preclude contaminants from falling into the quench bath.
 3. The method of claim 2 in which the controlled atmosphere is a high carbon potential carburizing gas, the purging gas is a low carbon potential gas, and in which the gas, fumes and other contaminants are exhausted from the chute at a rate equal to about two times the rate the purging gas is injected.
 4. The method of claim 3 in which the purging gas is injected into the chute at the rate of about 400 cubic feet per minute, and the gas, fumes, and other contaminants are exhausted at about 800 cubic feet per minute. 